FDCL is superior to other talents in that tier, cant imagine playing without it

Not definitively. If your just doing random BGs, or if your constantly the target of pressure - then FDCL gives you more free instant spells so that your damage is harder to shut down, in that case it's the best.

When your trying to burst your kill target down though, having Mindbender up every minute for that is fantastic - it's up 25% of the time and hits harder than Shadowfiend, which means that where CC'ing a Shadowfiend can be a good idea to shut down their mana return - CC'ing Mindbenders is pretty much a waste of time since they'll be back in a moment anyways: it distributes your risk of wasted/CC'd cooldowns.

So both of them are really good for different situations and playstyles - if your playing Shatterplay, then you don't want FDCL - you definitely want Mindbender for example, because you aren't going to be sat on if the mage is worth his salt, and that comp is all about maximizing burst on the swap - which Mindbender is way better than FDCL for. But if you were playing say, RPS (not that anyone plays RPS anymore), your very likely going to be the target the enemy team prefers to sit on - so FDCL would probably be a good bit more useful.

hardcore and high rating spriest will take insanity and twisted faith over divine insight and fdcl because these two talents with their burst and increased damage threshold will keep healers dead in their track from trying to heal up. insanity is definately worh the while. i'm glad i'm not alone in this forum backing it's potential usage in pvp and pve.

It's down to preferencial play to some people. the burst damage from fdcl and divine insight comes no where close to insanity and twisted faith working together. more chance for spike and mind blast instants sure but damage is lacking big time imo (will be too dependant on devouring plague) . in todays pvp those two talents will not keep a hard core healer under stress but the two i side with will keep them under pressure from retaliating.

insanity and devouring plague has the potential to hit over 100k like locks can do with their shadow bolt

mind blast and spike will not hit like that. more instants but weaker synergy

you can finish someone off @ 50% health with just 1 insanity proc and a 2x shadow word death thanks to twisted faith so yes i would skimp fdcl and divine insight.

i'm not saying it's bad i'm just saying the pressure isn't there against good healers and extremely potent against melee up in your face waiting for a direct heal to stun or silence. it great for kiting, dispelling and healing when VT mind blast mind spike isn't a good option for big burst.

the instants procs from mindspike and mind blast can be easily be out healed

Aside from the obvious counter of dispels if you go insanity, it limits your swapping potential.

Also, if you choose twisted fate you'll have a harder time getting an opponent under that threshold. I'm not saying its not a viable pick, but you'll have to make the judgment based on which situation you'd rather avoid:

a) Not putting enough pressure -> Go power infusion or DI
b) Not landing a kill when enemy is within execute range -> Go twisted fate
(If you're doing mass zergfest pvp twisted fate would probably be the way to go)